1. Field of the Invention
The present invention relates to a technique of effectively eliminating the potential for uneven printing caused by a positional misalignment of dots without any significant cost increase, in a printing device that creates dots on a printing medium to complete a printed image.
2. Description of the Related Art
Conventional printing devices in widespread use eject ink droplets from print heads and create dots on printing media to complete printed images.
One example of such printing devices is disclosed in Japanese Patent Laid-Open Gazette No. 8-132724.
In such printing devices, the manufacturing error of each print head may make the positions of dots actually created on a printing medium deviate from their expected positions and cause an uneven printing result. The uneven printing problem due to this positional misalignment of dots is described with reference to FIGS. 7 through 9. The following description regards an inkjet printer (hereafter simply referred to as the ‘printer’) as an example of such printing devices having the uneven printing problem.
FIG. 7 shows a typical arrangement of nozzle holes on a print head.
The print head has four nozzle arrays for respectively ejecting four color inks, black (K), cyan (C), magenta (M), and yellow (Y). Each nozzle array includes multiple nozzles arranged in two lines in a sub-scanning direction (not shown). Each nozzle in the nozzle array has one nozzle hole. The nozzle holes are thus grouped by the color and are arranged in the two lines in the sub-scanning direction as shown in FIG. 7. Each one-dot chain line represents a connecting line of the designed center positions of the aligned nozzle holes in each nozzle array.
In this illustrated example of FIG. 7, the manufacturing error of the print head makes the actual center positions of nozzle holes on a right line in the black (K) nozzle array deviate from the designed center positions. In the description below, the right line of nozzle holes having a deviation of the center positions is simply called the ‘nozzle holes H2’ and the left line of nozzle holes having no deviation of the center positions is simply called the ‘nozzle holes H1’.
FIGS. 8(A) and 8(B) show ink droplets ejected from the nozzle holes H1 and H2 of FIG. 7 to reach printing paper P.
FIG. 8(A) shows black (K) ink droplets ejected from the nozzle holes H1 to reach the printing paper P. FIG. 8(B) shows black (K) ink droplets ejected from the nozzle holes H2 to reach the printing paper P. The nozzle holes H1 and H2 of FIG. 8 are identical with the nozzle holes H1 and H2 explained above with reference to FIG. 7.
A print head of FIG. 8 mounted on the printer has main scans to move forward and backward in a main scanning direction and ejects ink droplets in both forward passes and backward passes. As shown in FIG. 8, black (K) ink droplets are ejected from the nozzle holes H1 in a forward pass and from the nozzle holes H2 in a backward pass.
The nozzle holes H1 have no positional deviation due to the manufacturing error and accordingly cause ejected ink droplets to reach their expected impact positions as shown in FIG. 8(A). The nozzle holes H2, on the other hand, have a positional deviation due to the manufacturing error and accordingly cause ejected ink droplets to reach positions deviated in the moving direction of the print head from their expected impact positions as shown in FIG. 8(B).
FIGS. 9(A) and 9(B) show resulting arrangements of dots created on the printing paper P of FIG. 8.
FIG. 9(A) shows an arrangement of dots created on the printing paper P on the assumption of no positional deviation of the nozzle holes H2. FIG. 9(B) shows an arrangement of dots created on the printing paper P on the assumption of a positional deviation of the nozzle holes H2 as shown in FIG. 7.
L1 to L8 of FIG. 9 represent raster lines (main scan lines). Dots on odd-numbered raster lines L1, L3, L5, and L7 are created in a forward pass of a main scan, whereas dots on even-numbered raster lines L2, L4, L6, and L8 are created in a backward pass of the main scan. The printer makes each sub-scan at a timing of changing the direction of main scan from the forward pass to the backward pass or at a timing of changing the direction of main scan from the backward pass to the forward pass. The sub-scan feeds the printing paper P in the sub-scanning direction that is perpendicular to the main scanning direction.
In the case of no positional deviation of the nozzle holes H2, dots are evenly scattered on the printing paper P to give an even printing result as shown in FIG. 9(A).
In the case of a positional deviation of the nozzle holes H2, on the other hand, ejection of ink droplets to form the dot pattern of FIG. 9(A) causes a leftward shift of the positions of dots on the even-numbered raster lines. The positional misalignment makes clear distinction between relatively dense dot areas and relatively sparse dot areas in the main scanning direction as shown in FIG. 9(B). This gives an uneven printing result including dark areas of relatively high dot densities and light areas of relatively low dot densities.
The cause of the uneven printing problem is not restricted to the manufacturing error of the print head, but the uneven printing problem is also ascribed to the attachment error of the print head and cockling of printing media due to ink absorption.
The printing device (printer) disclosed in the above cited reference of Japanese Patent Laid-Open Gazette No. 8-132724 is configured to prevent cockling of printing media as one main cause of the uneven printing problem. This configuration, however, requires a heat treatment device in addition to the general printer structure and thus significantly increases the total manufacturing cost of the printer.